1. Field of the Invention
The present invention relates generally to a position detecting method and a position detecting apparatus for detecting a position of a moving mechanism, using an incremental encoder coupled to the moving mechanism through a reduction gear and means for detecting division points in the moving mechanism, and more particularly, to a technique and method suitable for use in driving mechanisms for driving an arm-driven type or a stage-driven type of industrial robot.
2. Description of the Related Art
In the field of industrial robots, arm-driven type robots accompanying with rotary motion and stage-driven type robots accompanying linear motion are widely known and employed. In these types of robots, as, for example, in the arm-driven type, mechanisms accompanying rotary motion of the arm include a plurality of detecting devices are employed to detect angular position, each of which measure an absolute angle value of the output shaft of a reduction gear using, as a reference, an encoder attached at an input side of the reduction gear. In this case, in order to enable the use an inexpensive incremental encoder, it is necessary to initialize the relation between a known angle of the output shaft of the reduction gear and the corresponding output value of the incremental encoder. For this reason, as illustrated in FIG. 14, it is a common practice to perform a series of operations, known as "return-to-home position", wherein home position sensor 9 is located at a predetermined position within an operating or motion range of a robot arm 8 that is connected to output shaft 7 of the reduction gear. Arm 8 is at first moved to this position, as indicated by the arrow in FIG. 14. Then, arm 8 is moved to a position where a reference point on the incremental encoder is detected, whereby an angle of output shaft 7 is initialized to a known value. In order to provide a plurality of home positions within one turn of the output shaft, Japanese Patent Laid-Open No. 64-31209 proposes an arrangement in which an absolute encoder, in addition to an incremental encoder, is attached to the output shaft of the reduction gear to enable detection of the angle of the output shaft of the reduction gear through detection of minute angular movements of the shaft. Further, Japanese Patent Laid-Open No. 4-138503 proposes another method in which one revolution of the output shaft is divided into a plurality of different angular regions, and the ranges of the angular regions are measured with an incremental encoder so that measurement of the absolute angle value of the output shaft is accomplished.
On the other hand, in the above mentioned rotary type mechanisms, as in the case of the exemplified arm-driven mechanism shown in FIG. 14, there are cases where the rotary motion of output shaft 7 is restricted to a predetermined angular range because of a limited operating range for arm 8. In such a case, since the absolute angle of the output shaft is unknown without the initial return of arm 8 to the return-to-home position, a determination as to whether the current position of arm 8 is within the operating range or not cannot be determined based on information only from the incremental encoder. Thus, as illustrated in FIG. 15, an operating range detecting device is additionally employed to detect whether the output shaft is within the operating range. The operating range detecting device comprises a screen plate 4 which is fixed to the output shaft and has different circumferential regions corresponding to the operating range and non-operating range. An operating range detecting sensor 10, such as an optical sensor, is mounted relative to a screen plate 4 and is employed to detect the different circumferential regions of divisional points of screen plate 4 indicative of whether the detected position is within or without of the operating range. The arrow in FIG. 15 indicates the extent of the operating range so that the remaining region between the arrow heads is the non-operating range.
In case of performing return-to-home position operations in the conventional position detecting devices, mentioned above, since only one home position can be set in one revolution of the output shaft of the reduction gear, there is a problem that, depending upon the position of the output shaft at the start of the return-to-home position operation, a substantial amount of rotary motion may be necessary to move arm 8 into the home position defined by the position of the home position sensor. This necessitates long periods of time to accomplish the initialization operation of the device since a home position is required to be set for every complete rotation of the output shaft of the reduction gear. In addition, when this known technique is applied to a robot arm, there is a possibility that the arm collides with an obstacle which may exist within the range of movement of the arm.
In order to solve these problems, Japanese Patent Laid-Open No. 64-31209 and Japanese Patent Laid-Open No. 4-138503 propose methods in which a plurality of home positions are set within one revolution of the output shaft of the reduction gear. However, the method proposed in Japanese Patent Laid-Open No. 64-31209 requires an additional absolute value encoder attached to the output shaft and associated complex mechanisms which cause increased costs with decreased device reliability. On the other hand, in the method disclosed in Japanese Patent Laid-Open No. 4-138503, wherein the operation range of the output shaft is divided into different angular regions, there is a problem that, in order to increase the detection precision of an absolute angle of the reduction gear output shaft, it is necessary to correspondingly increase the detection precision of the division points, or to set a large angle between the different angular regions. In any case, it is not possible to reduce necessary manufacturing costs or to significantly reduce required rotary operations for the initialization operations.
Further, since the return-to-home position should be done within the angular operating range, at the beginning of the initialization operation, it is necessary to confirm, at the beginning of the initialization operation, that the arm is within its operating or motion range, and, if not, to first move the arm into the operating range. Therefore, it is necessary to provide another sensor, such as illustrated in FIG. 15, which correspondingly increases manufacturing costs and space requirements for the additional sensor, its means of attachment and its associated signal lines.
Furthermore, these various problems are not restricted to the above mentioned am-driven type robots employing rotary motion, but the same problems exist with linear motion positioning relative to stage-driven type robots.
The present invention is designed to solve these foregoing described problems with an objective of realizing a new position detecting apparatus and method for position detecting that reduces the required rotary motion range of the reduction gear output shaft and the linear motion range of a linear motion shaft of these detecting apparatus in achieving initialization, as compared to that conventionally employed, without increasing manufacturing costs and without sacrificing the initializing precision of the moving mechanism.
Another objective of this invention is the provision for restricting the operating or motion range of a position detecting apparatus, e.g., the rotary motion range of the reduction gear output shaft or the linear motion range of the linear motion shaft in such apparatus, and the utilization of a method for initializing the moving mechanism of the position detecting apparatus regardless of whether the home position is within the restricted operating range.